Laser tracker calibration system and methods

ABSTRACT

A system and method of calibrating a laser tracker is provided. The system includes a support system for quickly and easily moving an artifact to a desired position and orientation and for holding the artifact in the position and orientation. An adjustable alignment mirror is coupled to a first end of the artifact so that the more accurate ranging system of the laser tracker can be isolated to determine a reference length of the artifact. Additional measurements are then taken to exercise one or more error source within the tracker. The support system includes a positioner and a support beam for positioning and supporting the artifact. The artifact is coupled to the support beam using kinematic clamps that are designed to reduce or eliminate errors associated with over-constraining the artifact.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of each of thefollowing:

U.S. patent application Ser. No. 15/437,127, filed Feb. 20, 2017, nowU.S. Pat. No. 10,739,449, which is a continuation of U.S. patentapplication Ser. No. 14/796,683, filed Jul. 10, 2015, now U.S. Pat. No.9,575,163, which claims priority pursuant to 35 U.S.C. 119(e) to U.S.Provisional Patent Application Ser. No. 62/022,917, filed Jul. 10, 2014and titled “Laser Tracker Testing System”, the entire disclosures ofwhich are incorporated herein by reference; and

U.S. patent application Ser. No. 16/112,299, filed Aug. 24, 2018, nowU.S. Pat. No. 10,725,493, which claims priority pursuant to 35 U.S.C.119(e) to U.S. Provisional Patent Application Ser. No. 62/549,504, filedAug. 24, 2017 and titled “FOOT PEDAL ASSEMBLY”, the entire disclosuresof which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to calibration devices formeasuring equipment. More specifically, the present invention isconcerned with a system and method for calibrating high precisionmeasuring instruments, such as laser trackers and the like.

BACKGROUND OF THE INVENTION

A large number of industries require precise and accurate measuring fora number of applications such as production, manufacturing, and processcontrol. In many such applications, measurement errors on the order ofeven one ten-thousandth of an inch can be critical. Often, especiallywhen measuring the dimensions of large objects or a long distancebetween two objects, special equipment and/or instruments are used toachieve the necessary precision and accuracy for a particularapplication. Instruments such as laser trackers are particularly wellsuited for such applications because laser trackers are capable ofproviding extreme precision and accuracy, even when measuring thedimensions of large objects.

As is the case with calibration (or verifying proper calibration) ofvirtually any measuring instrument, checking calibration of a lasertracker is typically accomplished by measuring an object (such as alength reference artifact) of a standard, known length and confirmingthat the instrument measures the appropriate length. In particular, suchartifacts are used to verify whether a laser tracker instrument isyielding trustworthy results (i.e. demonstrating that it is properlycalibrated) or used during a measurement job to establish whetheranything has gone wrong during the course of the job. For example, auser of the instrument will measure the artifact at the beginning,middle, and end of a job. If the user obtains the same lengthmeasurement each time, the user will have a degree of comfort that thetracker has maintained a proper adjustment and/or position during themeasurement.

An acceptable method used to verify the proper calibration of a lasertracker is a length measurement system test. A length measurement systemtest involves several steps. First, two or more measuring points arelocated and oriented relative to a laser tracker. The measuring pointsare displaced a known distance from each other; the known distance beinga reference length. Next, the laser tracker measures the distancebetween each measuring point; the measured distance being a measuredlength. Finally, the measured length is compared with the referencelength so as to evaluate the performance of the laser tracker. Because alaser tracker may perform differently depending on the position andorientation of measuring points relative to the laser tracker, the abovesteps are repeated with the measuring points at various locations andorientations to exercise the various error sources within the tracker.

Prior to performing a length measurement system test, the referencelength must be established. One method of establishing a referencelength is described in Section D-3 of ASME B89.4.19-2006. Using thismethod, the measuring points are aligned with the laser tracker so thatthe distance between the measuring points may be measured with the moreaccurate ranging system of the tracker (such as an interferometer orAbsolute Distance Meter system). Another method of establishing areference length is to use a calibrated artifact.

A calibrated artifact includes a measuring point on or near each end ofthe artifact. Traditionally, artifacts were calibrated at a specifictemperature range and nominal thermal expansion corrections are providedto establish the reference length between the measuring points. Morerecently, several length reference systems have been developed thatinclude structural arrangements that are designed to counteract lengthincreases caused by thermal expansion. For example, U.S. applicationSer. No. 13/431,188, now U.S. Pat. No. 8,479,406, the entire disclosureof which is incorporated herein by reference, discloses a lengthreference bar system and method that compensates for thermal expansionand is capable of being calibrated using the method described in SectionD-3 of ASME B89.4.19-2006. Other length reference systems disclosed inU.S. Pat. Nos. 6,505,495, 6,836,323, and 7,188,428, the entiredisclosures of which are incorporated herein by reference, have alsobeen designed to counteract length increases caused by thermalexpansion. Still other length reference systems are fabricated frommaterials having low coefficients of thermal expansion, such as carboncomposite and/or high-grade invar, to further reduce the artifact'ssensitivity to temperature gradients.

When using an artifact to perform a length measurement system test, theartifact is positioned and oriented so as to move the measuring pointsto various locations and orientations relative to the laser tracker. Forexample, the artifact may be oriented vertically for a first test,horizontally for a second test, and diagonally for a third test. In eachorientation, the measuring points may be positioned symmetrically orasymmetrically relative to the laser tracker.

Precise and accurate movement of an artifact to a specific position andorientation is time consuming and difficult. Consequently, positioningrepeatability is also time consuming and difficult. Once in position,maintaining the position and orientation of the artifact can bedifficult. A fixture may solve some of these problems, but a fixturealso creates additional uncertainty with the accuracy of the referencelength.

To ensure that a length measurement system test is as accurate aspossible, the reference length must be as close as possible to theactual distance between the measuring points at the time the measuredlength is obtained. Unfortunately, several factors, such as “fixturingeffects,” create uncertainty as to the accuracy of a reference length“Fixturing effects” may include, but are not limited to, gravityeffects, loading effects, and mounting constraint effects. “Fixturingeffects” may be influenced by factors such as the straightness and/orstiffness of an artifact, the type, quantity, and/or location ofmounting fixtures, the location of targets relative to the mountingfixtures, potential vibration, and/or the accuracy of an alignment.Additionally, uncertainty of the artifact temperature and uncertainty ofthe coefficient of thermal expansion of the artifact material createuncertainty with the thermal expansion correction values (if used).

Fixturing effects may vary with temperature and/or with the orientationof the artifact. Accordingly, fixturing effects may be difficult todetect and/or to predict. For instance, an artifact at room temperatureand situated in a vertical orientation may experience negligiblefixturing effects while an artifact at twenty degrees above roomtemperature and situated in a horizontal orientation may experiencevarious fixturing effects such as thermal expansion restraint and/orcantilever bending. Thermal expansion restraint creates additionaluncertainty with the thermal expansion correction values. Cantileverbending creates additional uncertainty with the calibrated length of theartifact at various orientations.

Therefore, it is desirable to provide a reference length system andmethod that quickly moves measuring points into precise and accuratelocations and orientations relative to a laser tracker, thereby allowingfor positioning repeatability, that maintains an accurate referencelength, that is easy to manufacture, and that is simple to use.Furthermore, it is desirable to provide a system and method for quicklyand easily determining a reliable reference length between measuringpoints.

SUMMARY OF THE INVENTION

The present invention solves the foregoing problems through theutilization of a unique calibration and testing system as well as aunique method of using the calibration and testing system to orientmeasuring points relative to a laser tracker.

The present invention pertains generally to a laser tracker calibrationand testing system and a method of quickly and easily verifying theaccuracy of a laser tracker. Particularly, the present inventionpertains to positioning and holding measuring points, such as lasertargets, in a particular configuration. In one embodiment, a sphericallymounted retro-reflective laser tracker target (SMR) is coupled to anartifact such that positioning and holding the SMR in a particularconfiguration is accomplished by moving the artifact to, and holding theartifact in, a particular orientation.

The artifact is constructed from a material having a low coefficient ofthermal expansion. Additionally, the artifact is designed to berelatively lightweight and rigid so as to enable portability whileminimizing elastic deformations. For instance, in one embodiment theartifact is a carbon fiber composite beam. In another embodiment, theartifact is fabricated from a hollow rectangular beam.

The present invention also pertains to a support system and a method ofsupporting the artifact. In one embodiment, the support system includesa support beam and two kinematic mounts that are utilized to securelycouple the artifact to the support beam. In some such embodiments, thefirst and second kinematic mounts, combined, approximately represent aKelvin Clamp.

Some embodiments of the support system includes a ring brake and a yokethat is configured to receive the ring brake. In one such embodiment,the ring brake defines a first diameter and a flange extending from thefirst diameter defines a second diameter. In such an embodiment, theyoke defines a first slot for receiving the first diameter of the ringbrake and a second slot for receiving the flange of the ring brake. Inone such embodiment, the first slot of the yoke defines a bearingsurface in communication with the first diameter of the ring brake. Inanother such embodiment, the ring brake includes a ring brake bushingdefining an inner diameter and an outer diameter. In one suchembodiment, the inner diameter is in communication with the firstdiameter of the ring brake and the first slot of the yoke defines abearing surface in communication with the outer diameter of the ringbrake bushing.

Another embodiment of the support system includes a primary rod forselectively coupling a ring brake to a yoke when the ring brake isreceived by the yoke. In one such embodiment, the primary rod is biasedaway from the ring brake so that the ring brake may be readily receivedby the yoke. In another such embodiment, a second slot of the yokedefines a back surface and the ring brake defines an interface surfacethat is configured to interface with the back surface of the second slotof the yoke when the ring brake is selectively coupled to the yoke. Inone such embodiment, the interface surface is defined by one or moreraised surfaces.

Yet another embodiment of the support system includes a positionerhousing having first and second ends. In one such embodiment, a yoke iscoupled to the first end of the positioner housing and a primary rodextends from the second end of the positioner housing past the first endof the positioner housing so as to enable selectively coupling a ringbrake to the yoke. In one such embodiment, a hand wheel is coupled tothe primary rod at the second end of the primary housing.

One embodiment of the positioner housing includes an adjustable basemember that is capable of supporting the positioner housing at variousheights above the ground. Another embodiment of the positioner housingincludes a base member that is capable of coupling the positionerhousing to a support structure, such as a tri-pod, so as to enable theadjustable positioning of the positioner housing relative to the ground.

The present invention also pertains to an orientation and selectiverestraint system and a method for orienting and securely holding anartifact in a particular orientation. In one embodiment of theorientation and selective restraint system, a ring brake is rotatablycoupled to a yoke such that rotation of the ring brake relative to theyoke rotates the artifact relative to the yoke. In one such embodiment,a user may orient the artifact without touching the artifact. In thisway, the artifact may be oriented without introducing heat energy from auser's hand into the artifact.

Another embodiment of the orientation and selective restraint systemincludes a restraining mechanism to selectively restrain a ring brakefrom rotating relative to a yoke, thereby securely holding an artifactin a particular orientation. In one embodiment, the ring brake includesa flange defining a plurality of locating features for haptic feedbackand positioning repeatability. In one such embodiment, each locatingfeature is one of a tab, an indentation, and/or an aperture. In anotherembodiment, the restraining mechanism includes a plunger that is movablebetween an engaged configuration and a disengaged configuration. In theengaged configuration, the plunger is in communication with at least onelocating feature such that the ring brake is prevented from rotatingrelative to the yoke. In the disengaged configuration, the plunger isdisplaced from the locating feature such that the ring brake is allowedto rotate relative to the yoke. In one such embodiment, the plunger isbiased towards the engaged configuration.

The present invention also pertains to a portability system and a methodof quickly and easily moving the laser tracker calibration and testingsystem from one location to another. In one embodiment, the positionerhousing includes a handle to increase the portability. In anotherembodiment, the support beam includes at least one handle to increasethe portability of the support beam.

The present invention also pertains to an alignment system and a methodof aligning measuring points, such as target spheres, relative to alaser tracker. In one embodiment, each end of the artifact includes anaccessory tray that is configured so as to selectively receive analignment accessory. In one such embodiment, the alignment accessoriesare mounted on kinematics for repeatability. In another such embodiment,the alignment accessories are equally weighted and positionedsymmetrically to reduce and/or negate their effects on the referencelength.

In one embodiment, each alignment accessory is one of an alignmentmirror, an alignment target, or an alignment laser. In one suchembodiment, an alignment mirror and an alignment laser are selectivelycoupled to opposed ends of the artifact. In another such embodiment, analignment mirror and an alignment target are selectively coupled toopposed ends of the artifact.

In one method of the present invention, an alignment mirror isselectively coupled to a first end of an artifact and an alignment laseris selectively coupled to an opposed second end of the artifact. Thealignment laser and the alignment mirror are positioned such that thealignment laser projects a beam of light across the length of theartifact through the virtual centers of the target spheres onto areflective surface of the alignment mirror. The alignment mirror issteered so that the alignment beam is collinear with the laser trackerbeam and coincident with the laser tracker aperture. In this way, thealignment mirror and the alignment laser are capable of being used toquickly and easily align the beam of the laser tracker with the centersof the target spheres.

In another method of the present invention, an alignment mirror isselectively coupled to a first end of an artifact and an alignmenttarget is selectively coupled to an opposed second end of the artifact.A laser tracker beam is pointed at the alignment mirror and thealignment mirror is steered until the laser tracker beam projects acrossthe length of the artifact through the virtual centers of the targetspheres onto the alignment target. In this way, the alignment mirror,the alignment target, and the laser tracker beam are capable of beingused to quickly and easily align the beam of the laser tracker with thevirtual centers of the target spheres.

With nominal manufacturing tolerances, the present invention enablessufficient alignment to meet buck-in requirements to isolate the rangingsystem of the tracker. As a result, wear and tear on the tracker isreduced because the traditional procedure of re-locating the trackerin-line with the virtual centers of the targets is eliminated.

The foregoing and other objects are intended to be illustrative of theinvention and are not meant in a limiting sense. Many possibleembodiments of the invention may be made and will be readily evidentupon a study of the following specification and accompanying drawingscomprising a part thereof. Various features and subcombinations ofinvention may be employed without reference to other features andsubcombinations. Other objects and advantages of this invention willbecome apparent from the following description taken in connection withthe accompanying drawings, wherein is set forth by way of illustrationand example, an embodiment of this invention and various featuresthereof

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention, illustrative of the best modein which the applicant has contemplated applying the principles, is setforth in the following description and is shown in the drawings and isparticularly and distinctly pointed out and set forth in the appendedclaims.

FIG. 1 is an isometric view of an artifact of the present inventionbeing supported by a support system of the present invention and beingpositioned relative to a laser tracker so as to perform a method of thepresent invention.

FIG. 2 is a front view of the artifact and support system of FIG. 1,showing the artifact in a horizontal configuration.

FIG. 3 is a front view of the artifact and support system of FIG. 1,showing the artifact in an angled configuration.

FIG. 4 is a front view of the artifact and support system of FIG. 1,showing the artifact in a vertical configuration.

FIG. 5 is an exploded isometric view of the artifact and part of thesupport system of FIG. 1.

FIG. 6 is an exploded isometric view of a positioner of the presentinvention.

FIG. 7A is an isometric view of a second kinematic mount of the presentinvention, the second kinematic mount being in an assembledconfiguration.

FIG. 7B is an isometric view of the second kinematic mount of FIG. 7A,the second kinematic mount being in a disassembled configuration.

FIG. 7C is an exploded isometric view of the second kinematic mount ofFIG. 7A.

FIG. 7D is a side view of the second kinematic mount of FIG. 7A, thesecond kinematic mount being in an assembled configuration.

FIG. 7E is a side view of the second kinematic mount of FIG. 7A, thesecond kinematic mount being in a disassembled configuration.

FIG. 8A is an isometric view of a first kinematic mount of the presentinvention, the first kinematic mount being in an assembledconfiguration.

FIG. 8B is an isometric view of the first kinematic mount of FIG. 8A,the first kinematic mount being in a disassembled configuration.

FIG. 8C is an exploded isometric view of the first kinematic mount ofFIG. 8A.

FIG. 8D is a side view of the first kinematic mount of FIG. 8A, thefirst kinematic mount being in an assembled configuration.

FIG. 8E is a transparent side view of the first kinematic mount of FIG.8A, the first kinematic mount being in an assembled configuration.

FIG. 8F is a transparent side view of the first kinematic mount of FIG.8A, the first kinematic mount being in a disassembled configuration.

FIG. 9A is a front view of the artifact of FIG. 1.

FIG. 9B is a sectional view taken along line 9B-9B of FIG. 9A.

FIG. 9C is a sectional view taken along line 9C-9C of FIG. 9A.

FIG. 9D is a rear view of the artifact of FIG. 9A.

FIG. 10 is an exploded isometric view of the artifact of FIG. 1.

FIG. 11A is a front view of the artifact of FIG. 1.

FIG. 11B is an isolated view on an enlarged scale of a portion of FIG.11A.

FIG. 11C is a sectional view taken along line 11C-11C of FIG. 11A.

FIG. 11D is a sectional view taken along line 11D-11D of FIG. 11A.

FIG. 12A is an isometric view of an accessory tray of the presentinvention.

FIG. 12B is a front view of the accessory tray of FIG. 12A.

FIG. 12C is a side view of the accessory tray of FIG. 12A.

FIG. 12D is a side view of the accessory tray of FIG. 12A.

FIG. 13A is a front view of an artifact of the present invention showingan alignment mirror coupled to a first end of the artifact and analignment laser coupled to an opposed second end of the artifact.

FIG. 13B is a sectional view taken along line 13B-13B of FIG. 13A.

FIG. 13C is a sectional view taken along line 13C-13C of FIG. 13A.

FIG. 14A is a front view of an artifact of the present invention showingan alignment mirror coupled to a first end of the artifact and analignment target coupled to an opposed second end of the artifact.

FIG. 14B is a sectional view taken along line 14B-14B of FIG. 14A.

FIG. 14C is a sectional view taken along line 14C-14C of FIG. 14A.

FIG. 15A is an isometric view of an alignment mirror of the presentinvention.

FIG. 15B is a rear view of the alignment mirror of FIG. 15A.

FIG. 15C is a side view of the alignment mirror of FIG. 15A.

FIG. 15D is a sectional view taken along line 15D-15D of FIG. 15C.

FIG. 15E is an exploded sectional view of the alignment mirror of FIG.15A.

FIG. 16A is an isometric view of an alignment laser of the presentinvention.

FIG. 16B is a rear view of the alignment laser of FIG. 16A.

FIG. 16C is a side view of the alignment laser of FIG. 16A.

FIG. 16D is a top view of the alignment laser of FIG. 16A.

FIG. 16E is an exploded sectional view of the alignment laser of FIG.16A.

FIG. 17A is an isometric view of an alignment target of the presentinvention.

FIG. 17B is a rear view of the alignment target of FIG. 17A.

FIG. 17C is a side view of the alignment target of FIG. 17A.

FIG. 17D is a top view of the alignment target of FIG. 17A.

FIG. 17E is an exploded view of the alignment target of FIG. 17A.

FIG. 18A is a perspective view of the artifact of FIG. 13A showing abeam of light being directed from the alignment laser towards thealignment mirror and reflecting off of the alignment mirror away fromthe artifact.

FIG. 18B is a perspective view of the artifact of FIG. 18A, shown at adifferent angle as FIG. 18A.

FIG. 19A is a perspective view of the artifact of FIG. 14A showing abeam of light being directed towards the alignment mirror and reflectingoff of the alignment mirror towards the alignment target.

FIG. 19B is a perspective view of the artifact of FIG. 19A, shown at adifferent angle as FIG. 19A.

FIG. 20 is an exploded perspective view of a support beam and a ringbrake of the present invention.

FIG. 21A is a perspective view of a ring brake of the present invention.

FIG. 21B is a perspective view of a bushing of the present invention.

FIG. 22A is a perspective view of the bushing of FIG. 21B beinginstalled onto the ring brake of FIG. 21A.

FIG. 22B is a front view of the ring brake of FIG. 22A.

FIG. 22C is a sectional view taken along line 22C-22C of FIG. 22B, theview showing the bushing being installed on the ring brake.

FIG. 23A is a front view of the artifact and part of the support systemof FIG. 5.

FIG. 23B is a sectional view taken along line 23B-23B of FIG. 23A.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

As required, a detailed embodiment of the present invention is disclosedherein; however, it is to be understood that the disclosed embodiment ismerely exemplary of the principles of the invention, which may beembodied in various forms. Therefore, specific structural and functionaldetails disclosed herein are not to be interpreted as limiting, butmerely as a basis for the claims and as a representative basis forteaching one skilled in the art to variously employ the presentinvention in virtually any appropriately detailed structure.

Referring to FIG. 1, the laser tracker calibration and testing system 10of the present invention includes an artifact 100 and a support system20 for the artifact. In some embodiments, the artifact 100 is a hollowrectangular beam that is constructed from a light-weight but rigidcomposite material that has a low coefficient of thermal expansion. Inthis way, the artifact remains portable while minimizing deformations.In some such embodiments, the artifact 100 includes opposed front 112and rear 114 walls extending between first 102 and second 104 ends ofthe artifact 100 and opposed top 116 and bottom 118 walls extendingbetween respective top and bottom edges of the front 112 and rear 114walls so as to define an interior area 110. In other such embodiments,the artifact 100 includes end caps 106 coupled to each end of theartifact 100 so as to cap the hollow ends of the artifact 100.

As shown in FIG. 5, a preferred embodiment of the artifact 100 includesa plurality of laser target holders 150 coupled at or near each end ofthe artifact. In some embodiments, at least one laser target holder isalso coupled at or near a center point of the artifact 100. In theembodiment shown, some of the laser target holders are coupled to thefront wall 112 of the artifact 100 while other laser target holders arecoupled to the top wall 116 of the artifact so as to create a preferredlaser target holder configuration. It will be appreciated that otherembodiments of the artifact 100 include different laser target holderconfigurations.

Referring to FIGS. 5, 10, and 11A-14C, a preferred embodiment of theartifact 100 also includes at least one accessory tray 160. Theaccessory tray 160 includes a first interface 162 for securing theaccessory tray 160 to the artifact 100 and a second interface 164 forselectively receiving one or more accessory 60. In some embodiments, thefirst interface 162 includes a groove and/or a raceway that isconfigured to slide over one or more edge of one or more wall of theartifact 100. In some such embodiments, the front wall 112 of theartifact 100 defines a profile 126 at each end of the artifact 100, eachend profile 126 being configured to receive an accessory tray 160. Morespecifically, in some embodiments, each end profile 126 is configured soas to enable an accessory tray 160 to be slid into the end profile 126by aligning a groove 162 of the accessory tray 160 with one or more edgeof the end profile 126 and translating the accessory tray 160 so thatthe groove 162 of the accessory tray 160 receives the edges of the endprofile 126. In some such embodiments, an end cap 106 prevents theaccessory tray 160 from sliding out of the end profile 126, therebysecuring the accessory tray 160 in place relative to the artifact 100.

The second interface 164 of the accessory tray 160 is configured toselectively receive an accessory 60. In some embodiments, one or moreaccessory includes an alignment mirror 62, an alignment target 64, analignment laser 66, and/or some other alignment feature. In otherembodiments, the second interface 164 includes three indentationsdefined by an interface panel 166 of the accessory tray 160. In somesuch embodiments, each indentation is oval in shape and the indentationsare oriented relative to each other so as to form a Y-pattern. In somesuch embodiments, the accessory 60 includes three corresponding spheres70 that are configured to be simultaneously received by the threeindentations 164 so as to position the accessory 60 relative to theaccessory tray, thereby positioning the accessory 60 relative to theartifact 100. In some such embodiments, the spheres 70 and the interfacepanel 166 include complementary magnetic and/or metallic properties sothat the accessory 60 is selectively magnetically secured to theaccessory tray 160.

In a preferred embodiment, an accessory tray 160 is secured to each endof the artifact 100. In some embodiments, the accessories 60 are equallyweighted such that when an accessory 60 is selectively coupled to eachaccessory tray 160, the accessories 60 are positioned symmetricallyrelative to a support system 20 of the present invention so as to reduceand/or negate the effects on the artifact 100.

As shown in FIG. 5, the support system 20 includes a support beam 200.In some embodiments, the support beam 200 includes opposed front 212 andrear 214 walls extending between first 202 and second 204 ends of thesupport beam 200 and opposed top 216 and bottom 218 walls extendingbetween respective top and bottom edges of the front 212 and rear 214walls so as to define an interior area 210. In other embodiments, thesupport beam 200 includes end caps 206 coupled to each end of thesupport beam 200 so as to cap the hollow ends of the support beam 200.

As shown in FIGS. 9A-9C, the artifact 100 is selectively coupled to thesupport beam 200 by way of a first 500 and second 600 kinematic mount.The first kinematic mount 500 prevents the artifact 100 from rotatingabout a longitudinal axis of the artifact 100 and the second kinematicmount 600 prevents the artifact from rotating about the first kinematicmount 500. Additionally, the first kinematic mount 500 prevents theartifact from translating while the second kinematic mount 600 allowsfor differential thermal expansion and contraction of the support beam200 relative to the artifact 100 without stretching, compressing,twisting, and/or bending the artifact 100. In such an embodiment, thefirst 500 and second 600 kinematic mounts, combined, approximatelyrepresent a Kelvin Clamp.

In a traditional Kelvin Clamp, three traditional interfaces produce sixconstraints. A first traditional interface includes a first primarysphere and a cone that is configured to receive the first primary sphereso as to produce translational constraints in all three transitionaldegrees of freedom. A second traditional interface includes one point ofcontact between a second primary sphere and a first surface so as tocreate a first rotational constraint. Finally, a third traditionalinterface includes two points of contact between a third primary sphereand a second and third surface so as to produce two additionalrotational constraints. The second and third surfaces are positionedrelative to each other so as to create a “vee” shape.

As shown in FIGS. 8A-8F, the first kinematic mount 500 is movablebetween an assembled configuration and a disassembled configuration andincludes a first portion 510 having first 512 and second 514 primaryspheres and a second portion 520 having three auxiliary spheres 522 anda first surface 524. In the disassembled configuration, the first 510and second 520 portions are displaced from each other. In the assembledconfiguration, the first primary sphere 512 of the first portion 510nests against each of the three auxiliary spheres 522 of the secondportion 520 and the second primary sphere 514 of the first portion 510nests against the first surface 524 of the second portion 520. In thisway, when the first kinematic mount 500 is in the assembledconfiguration, the first 510 and second 520 portions of the firstkinematic mount 500 functionally represent a first and secondtraditional interface of a Kelvin Clamp, such that the first kinematicclamp 500 is capable of constraining the artifact 100 in four degrees offreedom relative to the support beam 200.

In some embodiments, the center point of the first primary sphere 512and the point of contact between the second primary sphere 514 and thefirst surface 524 define a pitch axis when the first kinematic mount 500is in the assembled configuration. In other embodiments, a lineperpendicular to the first surface and extending through the center ofthe first primary sphere 512 defines a yaw axis when the first kinematicmount 500 is in the assembled configuration.

In a preferred embodiment, the first kinematic mount 500 furtherincludes a first pressure point located at the point of contact betweena first pressure applicator 526 of the second portion 520 and a firstpressure surface 516 of the first portion 510. In some such embodiments,applying pressure at the first pressure point biases the first kinematicmount 500 towards the assembled configuration, thereby enabling thefirst kinematic mount 500 to restrain the artifact 100 in four degreesof freedom regardless of the orientation of the artifact. In other suchembodiments, the center point of the first sphere, the pressure point,and the point of contact between the second sphere and the first surfacedefine a pitch axis when the first kinematic mount 500 is in theassembled configuration.

As shown in FIGS. 7A-7E, the second kinematic mount 600 is movablebetween an assembled configuration and a disassembled configuration andincludes a first portion 610 having a first cylinder 612 and a secondportion 620 having two auxiliary spheres 622. In the disassembledconfiguration, the first 610 and second 620 portions are displaced fromeach other. In the assembled configuration, the first cylinder 612 ofthe first portion 610 nests against each of the auxiliary spheres 622 ofthe second portion 620. In this way, when the second kinematic mount 600is in the assembled configuration, the first 610 and second 620 portionsof the second kinematic mount 600 functionally represent a thirdtraditional interface of a Kelvin Clamp with the first cylinder 612functionally replacing the traditional structure of a third sphere andthe two auxiliary spheres 622 functionally replacing the traditionalstructure of a second and third surface positioned in a “vee” shape.

The second kinematic mount 600 includes a second pressure point locatedat the point of contact between a second pressure applicator 626 of thesecond portion 620 and the first cylinder 612 of the first portion 610.In some embodiments, applying pressure at the second pressure pointbiases the second kinematic mount 600 towards the assembledconfiguration regardless of the orientation of the second kinematicmount 600. Contact points between the first cylinder 612 and each of thetwo auxiliary spheres 622 define two tangent lines and a firstconvergence point. In some embodiments, the first convergence point iscoincidental with the longitudinal axis of the first cylinder 612. Inother embodiments, a tangent line passing through the second pressurepoint also passes through the first convergence point when the secondkinematic mount 600 is in the assembled configuration.

In a preferred embodiment, the first 500 and second 600 kinematic clampsare symmetrically biased towards respective first 102 and second 104ends of the artifact 100 such that, together, they are capable ofconstraining the artifact 100 in six degrees of freedom relative to thesupport beam 200. For instance, in some embodiments, the secondkinematic mount 600 is configured to restrain the artifact 100 in twodegrees of freedom, with the first constraint preventing the artifact100 from rotating about the pitch axis of the first kinematic mount 500and the second constraint preventing the artifact 100 from rotatingabout the yaw axis of the first kinematic mount 500.

The kinematic mounts are configured to constrain the artifact 100 in sixdegrees of freedom while minimizing or eliminating additionalconstraints. In some embodiments, the second kinematic mount 600 isconfigured to allow the first cylinder 612 to translate along alongitudinal axis of the first cylinder 612 so as to prevent thekinematic mounts from inducing or preventing elongation of the artifact100. In other embodiments, the longitudinal axis of the first cylinder612 of the second kinematic mount 600 defines a roll axis so as toprevent the kinematic mounts from inducing and/or correcting a twist inthe artifact. In still other embodiments, the longitudinal axis of thefirst cylinder 612 of the second kinematic mount 600 is coincidentalwith the center point of the first primary sphere 512 of first kinematicmount 500. In yet other embodiments, the longitudinal axis of the firstcylinder 612 of the second kinematic mount 600 is coincidental with thelongitudinal axis of the artifact 100.

In a preferred embodiment, as shown in FIGS. 9A-9D, the rear wall 114 ofthe artifact 100 defines first 122 and second 124 apertures that areconfigured to receive respective first 500 and second 600 kinematicmounts. In some embodiments, respective first portions 510, 610 of thefirst 500 and second 600 kinematic mounts are positioned at leastpartially within the interior area 110 of the artifact 100 andrespective second portions 520, 620 of the first 500 and second 600kinematic mounts are positioned at least partially outside of theinterior area 110 of the artifact 100 such that when the kinematicmounts are coupled to the artifact 100, each mount extends through itsrespective aperture. In other embodiments, each first portion 510, 610of the first 500 and second 600 kinematic mounts includes a firstmounting plate 518, 618 for selectively mounting respective kinematicmounts to the front wall 112 of the artifact 100.

In some embodiments, the front wall 212 of the support beam 200 definesfirst 222 and second 224 apertures that correspond with respective first212 and second 214 apertures of the artifact. Each aperture 222, 224 isconfigured to receive respective first 500 and second 600 kinematicmounts. In some embodiments, respective second portions 520, 620 of thefirst 500 and second 600 kinematic mounts are positioned at leastpartially within the interior area 210 of the support beam 200 andrespective first portions 510, 610 of the first 500 and second 600kinematic mounts are positioned at least partially outside of theinterior area 210 of the support beam 200 such that when the kinematicmounts are coupled to the support beam 200, each mount extends throughits respective aperture. In other embodiments, each second portion 520,620 of the first 500 and second 600 kinematic mounts includes a secondmounting plate 528, 628 for selectively mounting respective kinematicmounts to the rear wall 214 of the support beam 200.

In a preferred embodiment, as shown in FIGS. 9A-9D, the artifact 100 issecured to the support beam 200 by way of the first 500 and second 600kinematic mounts. In some embodiments, the mounts are symmetricallybiased towards respective first 102, 202 and second 104, 204 ends of theartifact 100 and the support beam 200 so as to equally balance theartifact 100 and the support beam 200 about a center point of thesupport beam 200. In some such embodiments, the kinematic mounts extendinto the interior areas 110, 210 of the artifact 100 and the supportbeam 200 such that the artifact 100 and the support beam 200 are capableof being positioned relatively adjacent to each other. In this way, thedistance between the rear wall 214 of the support beam 200 and thecenter of gravity of the artifact 100 is reduced and/or minimized.

In some embodiments of the support system 20, the support beam 200includes one or more handle 240 that is configured to enable a user tolift and/or otherwise move the artifact without requiring the user totouch the artifact. In this way, the artifact may be moved withoutintroducing heat energy from a user's hand into the artifact. In otherembodiments, the support system 20 further includes a positioner 300 anda base member 22 for supporting the positioner 300 above the ground. Insome such embodiments, the positioner 300 includes a handle 340. Inother such embodiments, the support beam 200 is rotatably coupled to thepositioner 300 such that by rotating the support beam 200 relative tothe positioner 300, the artifact 100 can be moved between a horizontalconfiguration, an angled configuration, and a vertical configuration. Inother such embodiments, the support beam 200 is selectively secured tothe positioner 300 such that the artifact is prevented from movingbetween configurations. In this way, the artifact can be moved betweenconfigurations and/or held in a particular configuration withoutrequiring a user to touch the artifact 100.

In a preferred embodiment, as shown in FIG. 20, the support system 20further includes a ring brake 250 and a yoke 350 that is configured toreceive the ring brake 250. In some embodiments, the ring brake 250includes a center web 260 and a ring flange 270 extending from anexterior surface 266 of the center web 260. In some such embodiments,the exterior surface 266 of the center web 260 defines a first diameterand an exterior surface 276 of the ring flange 270 defines a largersecond diameter. In other embodiments, the center web 260 defines acenter aperture 268.

In a preferred embodiment, as shown in FIG. 6, the yoke 350 includesopposed front 352 and rear 354 panels and a side panel 356 extendingbetween the front 352 and rear 354 panels so as to define an interiorarea 360 that is configured to receive at least part of the ring brake250. In some embodiments, the interior area 360 of the yoke 350 isconfigured to receive the ring flange 270 of the ring brake 250 and thefront panel 352 of the yoke 350 is configured to allow the center web260 of the ring brake 250 to extend through the front panel 352 of theyoke 350. In some such embodiments, the front 352 and side 356 panels ofthe yoke 350 define a top opening 362 such that the ring brake 250 canbe selectively received by the yoke 350 by sliding the ring brake 250through the top opening 362 of the yoke 350 and into the interior area360 of the yoke 350.

In a preferred embodiment, the front panel 352 of the yoke 350 defines acurved bearing surface 390. In some embodiments, the bearing surface 390of the yoke 350 is in communication with the exterior surface 266 of thering brake 250 when the ring brake 250 is received by the yoke 350. Inother embodiments, the ring brake 250 includes a bushing 290 having aninterior surface 292 defining an inner diameter and an exterior surface294 defining an outer diameter. The inner diameter is approximately thesame diameter as the first diameter defined by the exterior surface 266of the center web 260 and the outer diameter is larger than the innerdiameter but smaller than the second diameter defined by the exteriorsurface 276 of the ring flange 270. In some such embodiments, thebushing 290 is configured to receive at least part of the center web 260of the ring brake 250 such that the interior surface 292 of the bushing290 is in communication with the exterior surface 266 of the center web260. In some such embodiments, the bearing surface 390 of the yoke 350is in communication with the exterior surface 394 of the bushing 290when the ring brake 250 is received by the yoke 350.

Referring to FIGS. 5, 20, and 23B, the ring brake 250 is coupled to therear wall 214 of the support beam 200 with a front surface 252 of thering brake 250 contacting the rear wall 214 of the support beam 200 andthe ring flange 270 being displaced from the support beam 200 so as toprovide clearance for the front panel 352 of the yoke 350 to bepositioned between the support beam 200 and the ring flange 270 of thering brake 250 when the ring brake 250 is received by the yoke 350. Insome embodiments, the ring brake 250 is positioned relative to thesupport beam 200 and/or the artifact 100 such that when the ring brake250 rotates within the yoke 350, the support beam 200 and/or theartifact 100 rotates about a center point of the support beam 200 and/orthe artifact 100.

Referring to FIGS. 5 and 23B, the yoke 350 is coupled to a front end 312of a housing 310 of the positioner 300. In some embodiments, a proximalend 322 of a primary rod 320 extends through the rear panel 354 of theyoke 350 and couples to the ring brake 250 so as to selectively pull arear surface 254 of the ring brake 250 into frictional contact with therear panel 354 of the yoke 350 so as to secure the ring brake 250 to theyoke 350 and/or to orient the ring brake 250 relative to the yoke 350and/or the positioner 300. In some such embodiments, the proximal end322 of the primary rod 320 is configured to be received by the centeraperture 268 of the ring brake 250. In other such embodiments, the rearsurface 254 of the ring brake 250 is a raised surface so that tolerancesof the rear surface 254 can be more tightly controlled and/or so thatthe ring brake 250 can more easily rotate when the rear surface 254 ofthe ring brake is not in frictional contact with the rear panel 354 ofthe yoke.

In some embodiments, the positioner 300 includes one or more springmember 330 that is configured to bias the primary rod 320 away from thering brake 250 so that the ring brake 250 may be readily received by theyoke 350. In other embodiments, a distal end 324 of the primary rod 320extends through a rear end 314 of the housing 310 so as to more easilyallow a user to cause the primary rod 320 to engage or disengage withthe ring brake 250. In some such embodiments, a hand wheel 326 or someother similar mechanism is coupled to the distal end 324 of the primaryrod 320 so as to provide a user with a mechanical advantage whenengaging or disengaging the primary rod 320 with the ring brake 250.

Referring to FIG. 21A, the ring flange 270 of the ring brake 250 definesa plurality of locating features 272 for haptic feedback and positioningrepeatability. In some embodiments, each locating feature 272 is one ofa tab, an indentation, and/or an aperture. In other embodiments, theyoke includes a plunger 372 that is movable between an engagedconfiguration and a disengaged configuration. In the engagedconfiguration, the plunger 372 is in communication with at least onelocating feature 272 such that the ring brake 250 is inhibited fromrotating relative to the yoke 350. In the disengaged configuration, theplunger 372 is displaced from the locating feature 272 such that thering brake 250 is allowed to rotate relative to the yoke 350. In someembodiments, the plunger is biased towards the engaged configuration.

The present invention also pertains to a method of calibrating a lasertracker. In some embodiments, the method includes removing one or morelaser target 50 from an artifact 100 so as to enable a beam of light 80to be directed through the virtual centers of the laser targets 50. Somesuch embodiments further include coupling an alignment mirror 62 to afirst end 102 of the artifact 100 and coupling an alignment target 64 toan opposed second end 104 of the artifact 100. The alignment mirror 62is adjustable so that a beam of light 80 directed onto the alignmentmirror 62 can be manipulated to reflect the beam of light 80 towards thealignment target 64 through the virtual centers of the laser target 50.In some such embodiments, the alignment mirror 62 is adjustable relativeto a reference point on the reflective surface of the alignment mirror62 and the alignment target 64 includes a reference point thatcorresponds with the reference point of the alignment mirror 62 suchthat when the beam of light 80 is directed at the reference point of thealignment mirror 62 and the reflected beam of light 80 is directed atthe reference point of the alignment target 64, the reflected beam oflight 80 passes through the virtual centers of the laser targets 50.

In a preferred method, the artifact 100 is moved to a first positionrelative to a laser tracker and is oriented in a first configurationprior to performing a first test and/or taking a first set ofmeasurements. In some embodiments, the artifact 100 is horizontal in thefirst configuration and the center of the artifact 100 is locateddirectly in front of and at approximately the same height as a beamaperture 32 of the laser tracker 30 such that a beam of light 80emitting from the laser tracker aperture that is directed towards thealignment mirror is approximately horizontal. In other embodiments, theartifact 100 is angled or vertical in the first configuration and/or inone or more other configurations. In still other embodiments, the centerof the artifact 100 is not positioned directly in front of and/or is notpositioned at approximately the same height as the beam aperture 32 ofthe laser tracker 30.

In some embodiments, the beam of light 80 is directed at a referencepoint on a reflective surface of the alignment mirror 62 and thealignment mirror 62 is adjusted so as to reflect the beam of light 80towards a reference point of the alignment target 64. In some suchembodiments, one or more of the reference points is an alignment point,such as a point at the intersection of two or more lines. A laser target50 is then coupled to the artifact 100 at or near the second end 104 ofthe artifact 100 and the distance from the laser tracker 30 to thecenter of the laser target 50 by way of the alignment mirror 62 ismeasured. The laser target 50 is then moved to a position at or near thefirst end 102 of the artifact and a similar measurement is taken. Inthis way, the reference length of the artifact 100 can be quickly andeasily obtained using the more accurate ranging system of the lasertracker 30. In some embodiments, the direct distance from the lasertracker 30 and the center of the laser target 50 is also measured so asto exercise one or more error source within the tracker without movingthe artifact from the position and orientation in which the referencelength of the artifact 100 is established. In other embodiments, theartifact 100 is repositioned and/or reoriented so as to exercise one ormore other error source within the tracker. In some such embodiments, anew reference length is established for the artifact 100 after theartifact 100 has been repositioned and/or reoriented.

In other embodiments, an alignment mirror 62 is selectively coupled to afirst end 102 of an artifact and an alignment laser 66 is selectivelycoupled to an opposed second end 104 of the artifact 100. The alignmentlaser 66 and the alignment mirror 62 are positioned such that thealignment laser projects a beam of light across the length of theartifact through the virtual centers of the target spheres 50 onto areflective surface of the alignment mirror 62. The alignment mirror 62is then steered so that the alignment beam is collinear with the lasertracker beam and coincident with the laser tracker aperture 32. In thisway, the alignment mirror 62 and the alignment laser 66 are capable ofbeing used to quickly and easily align the laser tracker beam with thevirtual centers of the target spheres 50.

In the foregoing description, certain terms have been used for brevity,clearness and understanding; but no unnecessary limitations are to beimplied therefrom beyond the requirements of the prior art, because suchterms are used for descriptive purposes and are intended to be broadlyconstrued. Moreover, the description and illustration of the inventionsis by way of example, and the scope of the inventions is not limited tothe exact details shown or described.

Although the foregoing detailed description of the present invention hasbeen described by reference to an exemplary embodiment, and the bestmode contemplated for carrying out the present invention has been shownand described, it will be understood that certain changes, modificationor variations may be made in embodying the above invention, and in theconstruction thereof, other than those specifically set forth herein,may be achieved by those skilled in the art without departing from thespirit and scope of the invention, and that such changes, modificationor variations are to be considered as being within the overall scope ofthe present invention. Therefore, it is contemplated to cover thepresent invention and any and all changes, modifications, variations, orequivalents that fall with in the true spirit and scope of theunderlying principles disclosed and claimed herein. Consequently, thescope of the present invention is intended to be limited only by theattached claims, all matter contained in the above description and shownin the accompanying drawings shall be interpreted as illustrative andnot in a limiting sense.

Having now described the features, discoveries and principles of theinvention, the manner in which the invention is constructed and used,the characteristics of the construction, and advantageous, new anduseful results obtained; the new and useful structures, devices,elements, arrangements, parts and combinations, are set forth in theappended claims.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

What is claimed is:
 1. A laser tracker calibration system comprising: anartifact; a first kinematic mount selectively coupled to said artifactat a first location; and a second kinematic mount selectively coupled tosaid artifact at a second location, wherein said first and secondkinematic mounts are coupled to a rigid support structure so as to holdsaid artifact relative to a laser tracker during calibration of thelaser tracker.
 2. The laser tracker calibration system of claim 1,wherein said first and second kinematic mounts are configured toconstrain the artifact in six degrees of freedom so as to minimize oreliminate adverse effects associated with over-constraining theartifact.
 3. The laser tracker calibration system of claim 1, wherein:said first kinematic mount prevents said artifact from rotating about alongitudinal axis of said artifact; and said second kinematic mountprevents said artifact from rotating about said first kinematic mountand allows for differential thermal expansion and contraction of saidsupport beam relative to said artifact without stretching, compressing,twisting, and/or bending said artifact.
 4. The laser tracker calibrationsystem of claim 1, wherein: said artifact includes opposed front andrear walls extending between opposed first and second ends of saidartifact and opposed top and bottom walls extending between respectivetop and bottom edges of said front and rear walls so as to define aninterior area; said rear wall of said artifact defines first and secondapertures that are configured to receive respective first and secondkinematic mounts; and said first and second kinematic mounts extendthrough respective first and second apertures of said artifact such thatsaid first and second kinematic mounts extend into the interior areas ofsaid artifact so as to enable said artifact to be positioned moreclosely to the support structure.
 5. The laser tracker calibrationsystem of claim 1, further comprising accessory trays coupled to opposedfirst and second ends of said artifact, each accessory tray beingconfigured to receive one or more accessory.
 6. The laser trackercalibration system of claim 5, wherein one of said accessories is analignment mirror selectively coupled to said accessory tray at saidfirst end of said artifact and another of said accessories is analignment target selectively coupled to said accessory tray at saidsecond end of said artifact such that a beam of light reflected off ofthe alignment mirror can be directed towards the alignment target. 7.The laser tracker calibration system of claim 5, wherein said first andsecond ends of said artifact each define an end profile that isconfigured to receive an accessory tray, wherein said accessory traysinclude a first interface feature for securing said accessory trays tosaid end profiles of said artifact, and wherein said accessory traysinclude a second interface feature that is configured to selectivelyreceive an accessory.
 8. A method of calibrating a laser tracker, themethod comprising: utilizing a first laser target to obtain a firstmeasurement that is associated with a position of a first end of anartifact when the artifact is in a first orientation; utilizing analignment mirror and one of the first laser target or a second lasertarget to obtain a second measurement that is associated with a positionof a second end of the artifact when the artifact is in the firstorientation; and comparing the second measurement with the firstmeasurement so as to determine a first reference length of the artifact.9. The method of claim 8, further comprising utilizing the alignmentmirror to obtain the first measurement.
 10. The method of claim 8,further comprising configuring the alignment mirror prior to obtainingthe second measurement, such configuring step including: directing abeam of light towards a reflective surface of the alignment mirror sothat the beam of light reflects off of the reflective surface of thealignment mirror; adjusting the alignment mirror so as to direct thereflected beam of light towards the second end of the artifact.
 11. Themethod of claim 10, wherein the adjustment step causes the reflectedbeam of light to be directed towards an alignment target.
 12. The methodof claim 8, further comprising: orienting the artifact in a secondconfiguration; repeating the utilizing and comparing steps so as todetermine a second measured reference length of the artifact; comparingthe second measured reference length with the first measured referencelength.
 13. The method of claim 12, wherein the artifact is held inposition by a support system that includes one or more feature forassisting a user in quickly and easily moving the artifact from thefirst configuration to the second configuration.
 14. The method of claim8, further comprising: obtaining a third measurement that is associatedwith the position of the first end of the artifact; obtaining a fourthmeasurement that is associated with the position of the second end ofthe artifact; comparing the fourth measurement with the secondmeasurement so as to determine a first measured length of the artifact;and comparing the first reference length of the artifact with the firstmeasured length of the artifact, wherein the third and fourthmeasurements are obtained without utilizing the alignment mirror.
 15. Amethod of calibrating a laser tracker, the method comprising:positioning an artifact relative to the laser tracker, the artifacthaving an alignment mirror coupled to a first end of the artifact and analignment laser coupled to a second end of the artifact; orienting theartifact in a first configuration; directing a beam of light from thealignment laser towards a reflective surface of the alignment mirror sothat the beam of light reflects off of the reflective surface of thealignment mirror; adjusting the alignment mirror so as to direct thereflected beam of light at an aperture of the laser tracker; utilizing afirst laser target to obtain a first measurement that is associated witha position of the first end of the artifact; utilizing the alignmentmirror and one of the first laser target or a second laser target toobtain a second measurement that is associated with a position of thesecond end of the artifact; and comparing the second measurement withthe first measurement so as to determine a first reference length of theartifact.
 16. The method of claim 15, further comprising utilizing thealignment mirror to obtain the first measurement.
 17. The method ofclaim 15, further comprising: orienting the artifact in a secondconfiguration; repeating the directing, adjusting, utilizing, andcomparing steps so as to determine a second measured reference length ofthe artifact; comparing the second measured reference length with thefirst measured reference length.
 18. The method of claim 17, wherein theartifact is held in position by a support system that includes one ormore feature for assisting a user in quickly and easily moving theartifact from the first configuration to the second configuration. 19.The method of claim 15, further comprising: obtaining a thirdmeasurement that is associated with the position of the first end of theartifact; obtaining a fourth measurement that is associated with theposition of the second end of the artifact; comparing the fourthmeasurement with the second measurement so as to determine a firstmeasured length of the artifact; and comparing the first referencelength of the artifact with the first measured length of the artifact,wherein the third and fourth measurements are obtained without utilizingthe alignment mirror.